Surface base-coat formulation for metal alloys

ABSTRACT

Chromium-free coating composition with anti-corrosion and anti-fingerprint properties, particularly suitable for metal alloys, especially galvanized steel, and coated articles. Composition comprises aqueous-resin emulsion, hazardous air pollutant-free co-solvent, organo-functional silane, metal chelating agent, and chromium-free corrosion inhibitor, and optionally pH adjusting agent.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a coating for metal alloys. Morespecifically, the present invention relates to a chrome-free coating forprotecting metal alloys from deterioration and corrosion.

2. Background Art

It is well known in the art that galvanized steel must be protected fromoxidation. Various methods have been developed for protecting againstoxidation. Some examples include chromate treatment technology andelectrolytic chromate treatment technology. However, one problem withthese methods is that the methods are unable to form a chromate film onmetals simply by coating the metals and then drying.

Unlike reactive chromate treatment technology and electrolytic chromatetreatment technology, dry-in-place chromate treatment technology is ableto form a chromate film on metals simply by coating the metals and thendrying. As a result, a distinguishing feature of dry-in-place chromatetreatment is that it is not limited to particular metal substrates. As aresult, dry-in-place chromate treatment is frequently used to impartcorrosion resistance to metal surfaces to improve their adherence toresins and most importantly to improve paint adherence and post-paintingcorrosion resistance when painting is carried out.

At the present time, the main metals used in flat sheet structures arezinciferous-plated steel sheet, aluminum, and aluminum alloy flat sheet.These are widely used in such economic sectors as automotiveapplications, household electrical appliances, building materials, andso forth. These materials are almost inevitably subjected to a chromatetreatment due to contemporary demands for high added value.

A distinguishing feature of dry-in-place chromate treatment technologyis that it is not limited to particular metal substrates. However, thistechnology has other advantages. Because a desirable film is obtainedthrough just a simple application step, there is no specific requirementfor long reaction times, and simple equipment can be used, so that theline length can be reduced. Moreover, the effluent treatment load islight because a post-treatment water rinse is not required. Also,because dry-in-place films usually contain a higher proportion ofcorrosion-inhibiting hexavalent chromium than do reactive chromate andelectrolytic chromate films, dry-in-place chromate films can provide ahigher corrosion resistance than the other two types at the same add-onweight.

The corrosion-inhibiting hexavalent chromium is soluble in the water ofwet corrosive ambients. One drawback to dry-in-place chromate films isthat they are generally more soluble in water than reactive orelectrolytic chromate films. The main component exhibiting watersolubility in dry-in-place chromate films is the hexavalent chromiumions, and films exhibiting a high water solubility of this type aredenoted below as “low-fixed-chromium” films. As is well known, thehexavalent chromium ions are pollutants, and this fact has generallycreated demand for a sparingly water-soluble dry-in-place chromate filmhaving a high proportion of fixed or immobilized chromium.

In addition to the problem of environmental pollution, the lowproportion of fixed chromium in dry-in-place chromate films createsother problems for industrial application. One such problem is that thealkaline degreasing process elutes hexavalent chromium. A degreasingstep is generally required during the conversion of dry-in-placechromated metal stock into finished product. The degreasing step takesplace in downstream channels in order to remove contaminants, such asoil, dust, iron powder, and the like, that have been picked up duringvarious stages and of course during press forming. Since traditionalsolvent degreasing is in the course of being discontinued due to globalenvironmental issues, waterborne degreasing, such as alkalinedegreasing, normally must be employed for this purpose. The elution of aportion of the dry-in-place chromate film by alkaline degreasingrequires the installation of special effluent treatment facilities inorder to treat the hexavalent chromium ions in the spent degreasingbath.

Another problem occurs when waterborne resin coatings are applied ondry-in-place chromated stock. A very recent trend with flat sheet metalstock is that the stock is increasingly being painted with organic resinat the manufacturing stage in order to obtain various characteristicssuch as corrosion resistance, fingerprint resistance, lubricity, andinsulating characteristics. Again, in the case of organic resins,solvent-based resins are being replaced by waterborne resins for thesame environmental reason as above. The hexavalent chromium ions elutedfrom dry-in-place chromate coatings inhibit dispersion of the waterborneresin in such waterborne resin coatings. This either prevents normalapplication and formation of the resin coating or ends up gelling theresin coating bath itself.

The reasons outlined above have prompted strong demand for theappearance of a dry-in-place chromate treatment bath that provides asparingly water soluble film, i.e., a “high-fixed-chromium” film.

Dry-in-place chromate treatment baths generally take the form ofCr³⁺-containing aqueous chromic acid or dichromic acid solutions, andseveral methods have already been proposed that provide sparinglywater-soluble dry-in-place chromate films using such baths.

Japanese Examined Patent Application [Kokoku] Number Sho 61-58552[58,552/1986] discloses a method that uses a chromating bath based onchromic acid, chromic acid reduction product, and silica sol. However,the hexavalent chromium in the chromate film is still readily elutedwhen a surface-treated steel sheet bearing a chromate film formed bythis method is submitted, during processing and painting operations, toa pre-paint alkaline rinse. This causes the corrosion resistance of thefilm to decline.

Japanese Patent Application Laid Open [Kokai or Unexamined] Numbers Sho58-22383 [22,383/1983] and Sho 62-83478 [83,478/1987] teach the use of asilane coupling agent to reduce hexavalent chromium ions in the chromatetreatment bath. In each case the coatings afforded by these methods havean excellent paint film adherence. However, the chromate film affordedby the former method has a poor alkali resistance, because it is laiddown from a phosphoric acid-free chromate treatment bath. The chromatefilm afforded by the latter method also has a similarly inadequatealkali resistance.

Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho63-96275 [96,275/1988] teaches a treatment method that uses a chromatetreatment bath containing organic resin whose molecule has beenfunctionalized with specific amounts of hydroxyl group. The alkaliresistance is again often inadequate in this case because the organicresin in the chromate coating formed by this method contains carboxylmoieties produced by oxidation by chromic acid. In addition, thetreatment bath stability in this case is strongly impaired because thereaction of the hydroxyl-functional organic resin and chromic acidproceeds even in the treatment bath itself.

Japanese Patent Publication [Kokoku] Number Hei 7-33583 [33,583/1995]teaches a chromate treatment method that uses a chromate treatment bathcontaining carboxylic acid and/or a carboxylic acid derivative. Thischromate treatment bath affords only an inadequate improvement inapplication performance. In addition, because baking at 150° C. to 300°C. is required, this method entails substantial cost for its heatingfacilities, which runs counter to the current trend of economizing onenergy. Thus, drying temperatures not exceeding 100° C. are desirable inorder to fully exploit the overall merits of dry-in-place chromatetreatment systems.

As has been described above, the prior dry-in-place chromate treatmentbaths and treatment methods have suffered from a number of drawbacks,and a dry-in-place chromate treatment bath and treatment method thatwould be free of these drawbacks has remained heretofore unknown. Inother words, to date there has yet to appear a dry-in-place chromatetreatment bath and corresponding treatment method that provide a goodapplication performance and bath stability while also providing metalsurfaces with a sparingly water soluble chromate film with a good alkaliresistance, water resistance, corrosion resistance, and paint filmadherence.

Rust-proof properties have conventionally been imparted to cold-rolledsteel sheets, galvanized steel sheets, zinc-based alloy-plated steelsheets, and aluminum-plated steel sheets used for automobiles,electrical appliances, building materials, and the like, usually bycoating their surfaces with chromate layers. Chromating treatmentincludes electrolytic chromating and application chromating.Electrolytic chromating is accomplished, for example, by using a bathcomposed mainly of chromic acid and also containing other anions such assulfate, phosphate, borate, and halogens, for treatment of the metalsheet by cathodic electrolytic treatment. Application chromating isdesigned in consideration of the problem of elution of chromium fromchromated metal sheets, and it thus involves preparation of a treatmentsolution by adding an inorganic colloid or inorganic anion to a solutionwith a portion of the hexavalent chromium portion reduced to trivalentchromium beforehand or to a solution with a specified ratio ofhexavalent chromium to trivalent chromium, and immersing the metal sheettherein or spraying the metal sheet with the treatment solution.

Those chromate layers formed by electrolysis do not have sufficientcorrosion resistance despite the low elution of hexavalent chromium andthere is particular loss of corrosion resistance in cases whereconsiderable layer damage occurs during working, etc. On the other hand,while metal sheets coated with application chromated layers have highcorrosion resistance and especially high excellent corrosion resistanceof worked sections, elution of hexavalent chromium from the chromatelayers has been a problem. Elution of hexavalent chromium can beconsiderably reduced by coating with organic polymers, but this is stillinadequate. Although an improvement in reducing elution of hexavalentchromium can generally be achieved by a method known as resin chromatingtreatment, such as disclosed in Japanese Unexamined Patent PublicationNo. 5-230666, it is still impossible to avoid trace elution.

Thus, in order to completely inhibit elution of hexavalent chromium, itis necessary to develop a corrosion-resistant layer that uses absolutelyno hexavalent chromium.

One previous anti-corrosion technique for incorporating absolutely nohexavalent chromium is a method under development that uses anorganic-based corrosion inhibitor. The presently known organic-basedcorrosion inhibitors include carboxylates such as benzoates, azelates,etc. and compounds containing —S—, —N—(which readily interact with metalions), as well as complexes thereof.

Techniques for including organic-based corrosion inhibitors in layershave been proposed. Examples of such layers include the hydrooximecomplex of zinc disclosed in Japanese Unexamined Patent Publication No.62-23989, the metal chelate compounds of Mg, Ca, Ba, Zn, Al, Ti, Zr, Sn,Ni, etc. disclosed in Japanese Unexamined Patent Publication No.3-183790 and Japanese Unexamined Patent Publication No. 2-222556, thealkali earth metal salts, transition metal salts, and transition metalcomplexes of organic compounds disclosed in Japanese Unexamined PatentPublication No. 6-321851 and the titanium and zirconium complexes ofcarboxylic acids disclosed in Japanese Unexamined Patent Publication No.8-48916. These corrosion inhibitors, however, have weak anti-corrosioneffects due to the metal elements forming the complexes and thus havefailed to provide the same function as hexavalent chromium. Inparticular, almost no corrosion resistance can be expected at damagedsections or at the locations of layer defects produced during working.

Japanese Unexamined Patent Publication No. 7-188951 discloses a rareearth metal-organic chelate compound for the purpose of inhibitingcorrosion of metals that contact solutions, such as radiators or pipes.This corrosion inhibitor was designed as a water-soluble compound, toallow continuous provision of the corrosion inhibitor to corrosion sitesby circulation of the solution. Consequently, although the stronganti-corrosion effect of the rare earth metal element is utilized, withlayers on metal sheets wherein the absolute amount of corrosioninhibitor onto the corrosion sites is limited by the coating coverage,elution occurs out of the layer in humid atmospheres so that long-termcorrosion resistance comparable to chromate layers cannot be achieved.

An example of an anti-corrosive layer is a magnesium alloy materials.Magnesium alloy materials have the lightest weight among the practicalmetallic materials. Magnesium alloy materials also have a large specificstrength and a good castabilty. Wider application of the materials tocases, structural bodies, various parts, etc. of household appliances,audio systems, aircrafts, automobiles, etc. has been desired.Particularly, Al-containing AZ91D (Al: 8.3-9.7 wt. %) and AM60B (Al:5.5-6.5 wt. %) have a good fluidity in die-casting and thixo molding andthus are most desirable alloys.

However, magnesium has the most basic normal electrode potential amongthe practical metallic materials resulting in high corrosionsusceptibility when the metal is brought into contact with other metalsand a considerably poor anti-corrosiveness in an aqueous acidic,neutral, or chloride solution. For its application tocorrosion-excluding positions, e.g. good appearance-maintainingpositions etc., it is necessary to provide an anticorrosive treatment.The thin coat and conductive layer are preferred. Coatings are the mostpopular anti-corrosion means, but it is hard to apply coatings tomagnesium alloy materials per se because of the disadvantage that theresulting coating film has poor adhesiveness. Sometimes, corrosion mayoccur under the coating film, and thus it is the ordinary practice toconduct a substrate surface treatment in advance of the coating process.

The substrate surface treatment technology includes, for example,substrate surface treatments of forming a metal oxide film or asparingly soluble salt film by chemical conversion treatment oranodizing using such heavy metal oxo acid salts as chromates,permanganates, or phosphates so as to improve the corrosion resistanceand the adhesiveness of coating films. The chemical conversion coatingsgenerate a large amount of wastewater and toxic chemical contaminants.

It is also the ordinary coating practice to use oil paints and syntheticresin paints that contain lead compounds, zinc powder and its compounds,chromates, etc. as an anticorrosive pigment.

Processes for forming an anticorrosive film on a magnesium alloy aredisclosed in JP-A-9-176894 and JP-A-9-228062.

Surface treatments using specific chemical compounds such as chromatesand permanganates have problems relating to environmental friendliness,such as effluent water pollution problems and skin allergy problems foroperators. The use of such surface treatments is increasingly subject tostrict regulations. Phosphates are also more or less harmful to theenvironment and the corrosion resistance of resulting phosphate films isnot satisfactory. The salt (fog) spray test (ASTM B117) shows corrosionin 24 hours. Substitute processes for such substrate surface treatmentsare under development but these methods still have problems with respectto corrosion resistance.

Lead compounds or chromates contained as anticorrosive pigments incoating technology have problems relating to environmental friendliness.Furthermore, there are occasionally problems relating to corrosions.These problems are due to diffusion of oxygen or water generated bycorrosion present under the coating film or by coating film defects.

The invention disclosed in Japanese patent JP-A-9-176894 relates to anelectrolytic treatment. Anodizing requires a power source of highvoltage. An entirely uniform film is hard to obtain. The patentdiscloses treatments using an organometal that are highly reactive andthus an entirely uniform film is likewise hard to obtain.

It would be useful to develop an alloy coating that is easy to apply,environmentally friendly, and anti-corrosive. The preferred coating is awater-based system containing no carcinogenic chromates and no hazardousair pollutant (HAP) co-solvents.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a metal alloycoating composition, the coating having a chrome-free environmentallyfriendly formulation. The coating can be used as an anti-corrosioncoating and an anti-fingerprint coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and B are photographs showing two panels coated with thischrome- and HAP-free formulation were subjected to 120 hours of ASTMB-117 testing;

FIGS. 2A and B are photographs that display panels of 2024-T3Bare/Alodine 1200 (FIG. 2A) and on 2024-T3 Bare/AFP (FIG. 2B) coatedwith AD9318/AD2298 primer after a 1000 hour salt spray test; and

FIG. 3 shows the Bode-magnitude plots (frequency vs. impedance) ofAD9318/AD2298 coated on 2024-T3 Bare/AFP panels after soaking in 3% NaClsolution for 72 hours (♦) and 1000 hours (▴), and those on 2024-T3Bare/Alodine 1200 panels soaked for 72 hours (●) and 1000 hours (▪).

DESCRIPTION OF THE INVENTION

Generally, the present invention provides a metal alloy coating. Thecoating is an emulsion that contains only environmentally friendlymaterials. The coating is able to meet all quality control standardswith regard to electrical resistance, corrosion resistance, abrasionresistance, and adhesion to metal surface and topcoat (liquid and powderpaints).

The present invention provides a chrome-free, water-based, and hazardousair pollutant (HAPs)-free formulation for a pretreatment coating. Thecoating can be applied before any primers and topcoats are applied to asurface. These surface pretreatment coatings have excellent protectiveperformance (alkaline and salt spray resistances) for galvanized steel(electrogalvanized and hot-dip, and galvalume), magnesium alloys (suchas AZ31, AM20), titanium alloys (such as Ti-6Al-4V), and aluminum alloys(such as 2024-T3, 7075-T6), and other similar alloys for surfacepretreatment and corrosion protections. The galvanized steel, andmagnesium, titanium, and aluminum alloys are currently used in computer,cellular phone, notebook, bicycles, and aerospace industries. Thesesurface coatings also offer a superior property for adding primersand/or topcoats.

More specifically, the coating of the present invention includes thefollowing: 60-70% by weight of water, 15-25% by weight of resins, 10-20%by weight hazardous air pollutants (HAPs)-free co-solvents (such asdipropylene glycol normal butyl ether—DPnB, propylene glycol normalbutyl ether—PnB), 0.4-5% by weight organofunctional silanes (such asfunctionalized mercaptosilianes, functionalized aminosilanes,functionalized vinylsilanes), 0.1-1.0% by weight corrosion inhibitors,0.1-1.0% by weight metal chelating agents, and a trace amount of pHadjusting agents. A specific example of a surface base-coat formulationis as follows: the chrome-free, HAPs-free, and water-based emulsioncontains 94 g water, 32 g acrylic co-polymer resin, 25 g HAPs-freeco-solvents, 0.54 g chrome-free corrosion inhibitors, 0.2 g metalchelating agents, 0.27 g pH adjusting agents, and 3 g functionalizedsilanes.

The term “resin” includes, but is not limited to, acrylic emulsion,polyurethane emulsion, co-polymer emulsion, and other similar compounds.This list is included to exemplify the resins that can be used. The listis not intended to be exhaustive. Those of skill in the art knowadditional resins that are of a sub-micrometer or nanometer particlesize that can be utilized in the present invention.

The phrase “organofunctional silanes” as used herein is intended toinclude, but is not limited to, silanes that are sterically hinderedsubstituents located at silicon atoms. Preferably, the functional groupsare vinyl, epoxy, sulfur, amino, and other similar groups. This list isincluded to exemplify the organofunctional silanes that can be used. Thelist is not intended to be exhaustive. Those of skill in the art knowadditional organofunctional silanes that can be utilized in the presentinvention.

By “corrosion inhibitor” as used herein, the phrase is intended toinclude, but is not limited to, silicates, vanadates, metaborates,manganates, phosphates, mercapto-compounds, xanthic acid salts,dithiocarbamic acid salts, organic carboxylates, and other similarcompounds. This list is included to exemplify the corrosion inhibitorsthat can be used. The list is not intended to be exhaustive. Those ofskill in the art know additional corrosion inhibitors that can beutilized in the present invention.

The phrase “pH adjusting agent” as used herein is intended to include,but is not limited to, the following agents: ammonia, organic amines,and other similar agents. This list is included to exemplify thepH-adjusting agents that can be used, the list is not intended to beexhaustive. Those of skill in the art know additional pH adjustingagents that can be utilized in the present invention.

The term “protection” as used herein means that the coating compositionforms a layer inhibiting oxidation of the underlying surface andresisting the alkaline solution washing (or degreasing). The coating ofthe present invention is able to protect the underlying surface fromcorrosion. For example, when the coating of the present invention wasapplied to a surface, no corrosion is detected after 96 hours in salt(fog) spray test (ASTM B117). The coating shows no removal of paint filmafter subjecting to 2-3% trisodium phosphate solution at 65° C. for 3-5minutes.

The coating formulation of the present invention is preferably appliedto the surface of a metal alloy substrate using techniques known tothose of skill in the art. The preferred magnesium alloy substrate foruse in the present invention has excellent forgeability to form a thincasing with sharp bottom edges, corners and projections whose innersurfaces preferably have radii of curvature of about 2 mm or less,particularly about 1 mm or less. Preferably, the magnesium alloy used inthe present invention has a composition of 1-6 weight percent of Al, 0-2weight percent of Zn and 0.5 weight percent or less of Mn, the balancebeing substantially Mg and inevitable impurities.

When the amount of aluminum is less than one weight percent, themagnesium alloy has poor toughness, though it is well forgeable. On theother hand, when the amount of aluminum is more than six weight percent,the magnesium alloy has poor forgeability and corrosion resistance. Thepreferred amount of aluminum is two to four weight percent, particularlyabout three weight percent.

Zinc has similar effects as those of aluminum. From the aspect offorgeability and metal flow, Zn is preferably zero to two weightpercent. The preferred amount of Zn is zero to one weight percent.

If added in a small amount, magnesium functions to improve themicrostructure of the magnesium alloys. From the aspect of mechanicalproperties, magnesium is preferably 0.5 weights percent or less.

The magnesium alloy can contain other elements such as rare earthelements, lithium, zirconium, etc., in such amounts as not to adverselyaffect the forgeability, mechanical strength, etc. of the magnesiumalloys, usually in a total amount as small as 0.2 weight percent orless.

The magnesium alloys satisfying the above composition requirements arecommercially available as AZ31 (Al: about 3 weight percent, Zn: about 1weight percent, Mn: 0.2-0.3 weight percent, Mg and inevitableimpurities: balance), AM20 (Al: about 2 weight percent, Mn: about 0.5weight percent, Mg and inevitable impurities: balance), etc., in ASTM.

The magnesium alloy body is preferably formed into a thin forged casingby at least two steps. In a preferred embodiment, the forging comprisesa first forging step and a second forging step. If necessary, a furtherforging step can be added between the first and second forging steps.

The first forging step involves shaping the body. The magnesium alloybody can be in any shape such as rectangular parallelepiped, cylinder,etc., as long as it is forgeable to a desired shape. However, it hasbeen found that when the magnesium alloy body is in a thick bulk shape,the resultant forged product has flow marks on the surface. The term“flow marks” means marks indicating traces of plastic flow of themagnesium alloy occurring during the forging process.

When a thin magnesium alloy body is forged at a low compression ratio,the flow marks can be suppressed, because disturbed plastic flow doesnot occur at a low compression ratio. The term “compression ratio” usedherein means a ratio (percentage) expressed by the formula:[(t₀-t_(f))/t₀]×100%, wherein t₀ is an original thickness of themagnesium alloy body to be forged, and t_(f) is a thickness of theforged product.

A compression ratio is preferably within 75% in the first forging stepand within 30% in the second forging step to sufficiently suppress theflow marks on the resultant thin forged casings. To achieve the abovecompression ratios, the magnesium alloy body is preferably in a thinplate shape having a thickness of about 3 mm or less. With such a thinmagnesium alloy plate, the mechanism of plastic flow can be utilized toproduce a thin forged casing with no flow marks. Because the originalsurface conditions of the magnesium alloy plates are substantially kepton the forged products, it is preferable to use the magnesium alloyplates with extremely small surface roughness. In the case of a roundmagnesium alloy rod, the compression ratio can usually be more than 80%.

In the case of forming a forged casing of about 1.5 mm or less inthickness, with an anodic oxidation coating for exhibiting metallicglow, it is important to forge a thin magnesium alloy plate of about 3mm or less, preferably about 2 mm or less, particularly about 1-1.5 mmin thickness.

Though the size of the magnesium alloy plate can be determined dependingon the compression ratio, it is preferable that the magnesium alloyplate is equal to or slightly larger than a bottom area of the finalthin forged casing. When the magnesium alloy plate is too large, theresultant thin forged casings are likely to have wrinkles at bottomedges and corners, lowering the yield of the final products. On theother hand, when the magnesium alloy plate is too small, the resultantthin forged casings are unlikely to be uniform in thickness inperipheries.

The magnesium alloy body to be forged is first preheated uniformly at atemperature of 350-500° C., slightly higher than the forging temperatureof the magnesium alloy body. The preheating temperature of the magnesiumalloy body is defined herein as a temperature of atmosphere inside anelectric furnace in which the magnesium alloy body is heated.

If the preheating temperature is lower than 350° C., the magnesium alloydoes not smoothly flow into the die cavity during the forging process,thus failing to make the thickness of the resultant forged casing assmall as about 1.5 mm or less. If the preheating temperature is higherthan 500° C., the magnesium alloy body is totally or partly melted,resulting in extreme metal flow marks appearing on the surface, whichmakes it impossible to obtain a thin forged casing with high quality.Also, a higher temperature causes excessive oxidation and even burningof the magnesium alloy during the forging process. The preferredpreheating temperature of the magnesium alloy body is 350-450° C.,particularly 400-450° C.

If the magnesium alloy body is heated in the air, a surface of themagnesium alloy body is severely oxidized, adversely affecting theforgeability, corrosion resistance, and surface appearance of theresultant thin forged casing. The preheating of the magnesium alloy bodyis carried out in vacuum or in an inert gas atmosphere such as an argongas, etc.

The preheating time is determined depending on the size of the magnesiumalloy body. For instance, it is about 10-20 minutes for a cylindricalmagnesium alloy body of 30 mm in diameter and 10-30 mm in length. If themagnesium alloy body were in a thin plate shape of about 3 mm or less inthickness, the preheating time would be sufficient to be as short as5-15 minutes.

The first forging step can be carried out on the magnesium alloy bodyunder conditions of a die temperature of 350-450° C., a compressionpressure of 3-30 tons/cm², a compressing speed of 10-500 mm/sec, and acompression ratio of 75% or less.

The die temperature is almost equal to the first forging temperature.When the die temperature is lower than 350° C., the preheated magnesiumalloy body is so cooled by contact with the die that sufficient metalflow cannot be achieved during the first forging step, resulting inrough forged surface. On the other hand, when the die temperature ishigher than 450° C., the forged product cannot easily be removed fromthe die. The preferred die temperature is 360-420° C. The first forgingtemperature is about 50-80° C. lower than a temperature at which themagnesium alloy starts melting to prevent the magnesium alloy frommelting locally during the first forging step.

The pressure at which the magnesium alloy body is compressed by a pairof die blocks is 3 tons/cm² or more. When the compression pressure isless than 3 tons/cm², the resultant intermediate forged product cannotbe made fully thin. The upper limit of the compression pressure canusually be determined based upon the compression ratio. Too highcompression pressure causes damage to the edges of the die. In addition,even though the compression pressure exceeds 30 tons/cm², furtherimprovements in the quality of the forged products cannot be obtained.The upper limit of the compression pressure is 30 tons/cm². Thepreferred compression pressure in the first forging step is 5-25tons/cm².

The compressing speed of the magnesium alloy body can be 10-500 mm/sec.When the compressing speed is less than 10 mm/sec, the productivity ofthe intermediate forged products is too low. When the compressing speedis more than 500 mm/sec, metal flow cannot follow the compression of themagnesium alloy body, resulting in disturbed metal flow, which leads toextreme flow marks on the surface of the body. The preferred compressionspeed in the first forging step is 50-300 mm/sec.

The compression ratio is preferably within 75% in the first forging stepto sufficiently suppress the flow marks on the resultant intermediateforged products. If the compression ratio exceeds 75%, it is difficultto prevent the flow marks from appearing on the surfaces of theresultant intermediate forged products. The more preferred compressionratio in the first forging step is 50-50%, particularly 18-45%.

The forging can be carried out mechanically or hydraulically.

The second forging step includes preheating the intermediate forgedproduct. The intermediate forged product obtained in the first forgingstep is preheated uniformly at a temperature of 300-500° C. in vacuum orin an inert gas atmosphere such as an argon gas, etc. If the preheatingtemperature of the intermediate forged product is lower than 300° C.,smooth metal flow does not occur along the cavity surface of the forgingdie during the second forging step, thereby failing to preciselytransfer the cavity surface contour of the second forging die to thefinal thin forged casing. If the preheating temperature is higher than500° C., the intermediate forged product can be melted in portionssubjected to strong friction, resulting in extreme flow marks appearingon the surface. The preferred preheating temperature of the intermediateforged product is 350-450° C.

The preheating time of the intermediate forged product is alsodetermined based upon the size of the intermediate forged product. Forinstance, it is about 5-15 minutes for the intermediate forged productof 1 mm in thickness.

The second forging step is preferably carried out on the intermediateforged product under the conditions of a die temperature of 300-400° C.,a compression pressure of 1-20 tons/cm², a compressing speed of 1-200mm/sec., and a compression ratio of 30% or less.

The die temperature is almost equal to the second forging temperaturebut can be slightly lower than the first forging temperature because thecompression ratio is smaller in the second forging step than in thefirst forging step. When the die temperature is lower than 300° C., thepreheated intermediate forged product is so cooled by contact with thedie that cavity surface contour cannot be precisely transferred from thesecond forging die to the resultant thin forged casing by the secondforging step. When the die temperature is higher than 400° C., theforged product cannot easily be removed from the die. Therefore, thepreferred second die temperature is 330-400° C.

The compression pressure in the second forging step can be smaller thanin the first forging step, and is preferably 1-20 tons/cm². When thecompression pressure is less than 1 tons/cm², the resultant forgedcasing cannot be made fully thin with excellent surface contour. Whenthe compression pressure exceeds 20 tons/cm², further improvements inthe quality of the forged products cannot be obtained. The preferredcompression pressure in the second forging step is 5-15 tons/cm².

The compressing speed of the intermediate forged product can be 1-200mm/sec. When the compressing speed is less than 1 mm/sec, theproductivity of the forged casings is too low. When the compressingspeed is more than 200 mm/sec, the cavity surface contour of the secondforging die cannot be precisely transferred to the thin forged casing,failing to provide the thin forged casing with excellent surfaceconditions. The preferred compression speed in the second forging stepis 20-100 mm/sec.

The compression ratio is preferably within 30% in the second forgingstep to sufficiently suppress the flow marks on the resultant thinforged casings. If the compression ratio exceeds 30%, it is difficult toprevent the flow marks. from appearing on the surfaces of the resultantthin forged casings. The more preferred compression ratio in the secondforging step is 5-20%.

In one embodiment, the thin forged casing of the present invention canbe a box-shaped, thin plate that has projections of various heights oneither or both surfaces. The thickness of the thin plate in areaswithout projections is preferably as small as about 1.5 mm or less, morepreferably about 1 mm or less. The projections can be bosses for screwholes, projections indicating alphabets, numbers and/or symbols, etc. Ofcourse, the thin plate portion can have thinner regions than theremainder unless the thinner regions affect the mechanical strength ofthe thin forged casing.

The thin forged casing of the present invention preferably has sharpbottom edges, corners and projections. Particularly in the case of smallcasings, for instance, those of minidisks, the inner surfaces of bottomedges and corners preferably have radii of curvature of 1 mm or less.Sharp bottom edges, corners, and projections whose inner surfaces havesuch small radii of curvature can be provided only by the forging methodof the present invention.

The resultant thin forged casing is trimmed at sidewalls by a cutter,etc. such that the sidewalls have exactly the same height. If necessary,screw bores can be formed in the boss projections. The thin forgedcasing can then be polished.

After polishing, the thin forged casing is subjected to a surfacecoating such as an anodic oxidation coating, a paint coating, etc. andthe anti-corrosion coating of the present invention. In the presentinvention, a chrome-free and HAPs-free aqueous emulsion is preferred.The coating can be applied in any manner known to those of skill in theart. Examples of such techniques include, but are not limited to,spraying the coating on the surface to be coated, dipping the surface inthe coating composition, and painting the coating on the surface to becoated.

The anodic oxidation coating can be applied using methods known to thoseof skill in the art. An electrolytic solution for anodic oxidation canhave a composition comprising one or more of sodium dichromate, acidicsodium fluoride, acidic potassium fluoride, acidic ammonium fluoride,ammonium nitrate, sodium dihydrogenphosphate, ammonia water, etc. Theelectrolytic components can be combined depending on the composition ofthe magnesium alloy, the desired color of the thin forged casing, etc.

Because the anodic oxidation coating is generally transparent with orwithout tint, the anodized thin forged casing keeps metallic glossinherent in the magnesium alloy.

Though the paint coating can be applied with any paint, it is preferableto coat a clear paint if metallic gloss is desired. The clear paint canbe made of thermosetting acrylic resins, polyester resins, epoxy resins,etc. without or trace of pigments like clear coatings of automobiles,etc. Before coating, the thin forged casing is preferably subjected to asurface base-coat of aqueous emulsion with the anti-corrosive treatmentof the present invention. The anti-corrosive coating is applied as asingle coat on the surface of the magnesium alloy. The coated alloy isthermally cured at approximately 125° C. for three to five minutes. Thedry film of final coating is approximately 1.6 μm thick with aresistance of approximately 0.3Ω. The coating provides an excellentmetal surface (and top coat) adhesion and can pass a salt spray testof >72 hours that is superior to those chemical conversion coatingsknown in the prior art.

The invention is further described in detail by reference to thefollowing experimental examples. These examples are provided for thepurpose of illustration only, and are not intended to be limiting unlessotherwise specified. Thus, the invention should in no way be construedas being limited to the following examples, but rather, should beconstrued to encompass any and all variations which become evident as aresult of the teachings provided herein.

EXAMPLES Example 1

A chemically pretreated galvanized or zinc-alloy-plated steel sheet iscommonly used to inhibiting corrosions of steel substrates. Theprocessed galvanized steel has poor fingerprint resistance and earthingproperties. The corrosion inhibition galvanized steel is also poor,leading to the formation of white rust covered the entire zinc coatedsteel in less than 24 hours in a salt (fog) spray test (ASTM B-117). Thegalvanized steel is utilized in large quantity for electronic parts,equipment or the like that require good fingerprint resistance, earthingproperties, and corrosion resistance. In current industrial practice,the ultra thin organic coatings (about 1 micrometer thick) are generallyapplied on high-speed lines. This desired organic coating should haveexcellent anti-fingerprinting characteristics, resist to alkalinesolution (i.e. 2% tri-sodium phosphate solution at 65 degrees C. for 2minutes) and passes a 120 hours salt (fog) spray test (ASTM B117). Thesechrome- and HAP-free anti-fingerprint coatings have been tested atdifferent independent laboratories, and have been shown to pass bothalkaline solution washing and 120 hours salt (fog) spray tests. Twopanels coated with this chrome- and HAP-free formulation were subjectedto 120 hours of ASTM B-117 testing at the China Steel Corporation andare shown in FIGS. 1A and B. FIG. 1A was tested without alkalinesolution washing, and FIG. 1B was tested after 2 minutes of alkalinewashing at 65° C.

The most commonly used coating formulation in today's industrialpractice is a water-based organic composite coating that contains water,resins (acrylic. emulsion, polyurethane emulsion, co-polymer emulsion,etc.), isopropyl alcohol (co-solvent), and a large quantity ofhexavalent chromates (corrosion inhibitors). This coating formulationworks extremely well, but the hexavalent chromates are toxic andcarcinogenic that cause lung cancer, and kidney and liver damage, andisopropyl alcohol is considered as hazardous air pollutants (HAPs). WhenOSHA implements the projected stringent limits within the next fewyears, many chromate prier end-users will find it difficult to complywith worker exposure limits and environmental safety.

The competition and challenge started several years ago for developingthe chrome-free and HAPs-free water-based anti-fingerprint coatings ongalvanized steel for using on high-speed lines. There is no satisfactoryformulation known currently that passes the required properties and thatare chrome-free and HAPs-free. This invention is described for the firsttime, chrome-free and HAPs-free water-based anti-fingerprint coatingsthat pass all required tests. The coating formulations contain 60-70% byweight of water, 15-25% by weight of resins (sub-micrometer or nanometersize resin particles, e.g. acrylic emulsion, polyurethane emulsion,co-polymer emulsion, etc.), 10-20% by weight of HAPs-free co-solvents,0.5-5% of organofunctional silanes sterically hindered substituents atsilicon atoms (the functional groups are vinyl, epoxy, sulfur, amino,etc.), 0.1-1.0% corrosion inhibitors (silicates, vanadates, manganates,phosphates, organic carboxylates, etc.) and a trace amount of pHadjusting agents (ammonia, organic amines, etc.).

Example 2

Magnesium-based alloys are of interest for many industrial applicationsdue to their favorable strength to weight ratio, such as AZ91 and ZE41.However, it is the corrosion resistance that often limits theapplications of magnesium-based alloys. Furthermore, the surface of amagnesium alloy is known to be very difficult to coat. Even with thechromic acid (toxic and carcinogenic) treatment applied it causesserious problems such as insufficient adhesive strength resulting from arelease agent and unevenness of treatment involved and inadequatecorrosion resistance incurred from slight impurities contained in thematerials.

The current surface treatment processed for magnesium alloys arechromate conversion coating non-chromate (i.e. manganate, vanadate,stannate, etc.) conversion coating, cold phosphate conversion coating,and galvanic anodizing treatment. The processes involve multiple stepsand are error-prone and costly. The multi-step surface treatmenttechnologies produce waste including organic solvents, heavy metals, andother toxic and deleterious materials.

Applicants have used a green chemistry approach and developed an aqueousemulsion coating for surface treatment of magnesium alloys. The emulsioncontains only environmentally safe chemicals, and precursors hybridizedof acrylic co-polymers and silanes.

A single-coat application of “Acryl-Mg-Sol” on magnesium alloy surface,followed by a thermal curing at 150 degrees C. for 5 minutes has shownto give a dry film thickness of approximately 1.6 μm with a resistanceof ˜0.3 mΩ/cm. The protective film displays an excellent metal surfaceadhesion (5B, ASTM D3359), and has passed a salt (fog) spray test (ASTMB117) of >24 hours that is superior to a multi-step chrome (ordichromate) pickle treatment.

Example 3

The AFP (anti-fingerprint coating) was developed recently in applicant'slab. It has been shown to provide excellent metal surface pretreatmenton bare cold-rolled steel (CRS), galvanized steel, magnesium alloys, andtitanium alloys. Here, the AFP system is extended and applied to theuntreated 2024-T3 Bare Al coupon, by dipping and spinning off the excessemulsion. The pretreated Al coupon is then thermally cured at 150° C.(oven temperature) for 1 min. to give a treated 2024-T3 Bare/AFP Alpanel. A 0.8-0.9 mil dry film of AD9318/AD2298 chromate primer wasprepared on 2024-T3 Bare/AFP and 2024-T3 Bare/Alodine 1200 coupons, andcured overnight at 49° C. The resistance to corrosion of AFP and Alodine1200 surface pretreatment on 2024-T3 Bare aluminum alloy is examined bysalt spray tests and electrochemical impedance spectroscopy (EIS) scans.

FIG. 2 displays panels of 2024-T3 Bare/Alodine 1200 (photograph A) andon 2024-T3 Bare/AFP (photograph B) coated with AD9318/AD2298 primerafter a 1000 hour salt spray test. Both panels are free of white rust,field blisters, white pits, or other undesirable defects. The photographshown in FIG. 2A (Alodine 1200 panel) shows stains along the X-scribearea.

The photograph shown in FIG. 2B (AFP panel) is free of stain. A slightdiscoloration (i.e., a leaching of chromate anti-corrosive pigments) isobserved in FIG. 2A, but not in FIG. 2B. This is an importantobservation, because the non-chromate AFP surface pretreatment retainsthe chromate anti-corrosive pigments in the primer, while the Alodine1200 pretreatment does not. The ability of AFP to retain the chromatepigments in the primer film will prolong the effect of corrosionresistance and, more importantly, reduces the possibility of chromatecontaminations of the groundwater and environment.

The salt spray testing results are in good agreement with the EISmeasurements. FIG. 3 shows the Bode-magnitude plots (frequency vs.impedance) of AD9318/AD2298 coated on 2024-T3 Bare/AFP panels aftersoaking in 3% NaCl solution for 72 hours (♦) and 1000 hours (▴), andthose on 2024-T3 Bare/Alodine 1200 panels soaked for 72 hours (●) and1000 hours (▪). The paint film of AD9318/AD2298 coated on 2024-T3Bare/AFP panel has a slope of nearly −1, indicating to a pure capacitor,with a high impedance value of 4×10⁹ Ω·cm² at 0.01 Hz (♦). This highquality of paint film protective performance is completely retainedafter soaking in 3% NaCl solution for 1000 h (▴). On the other hand, thepaint film of AD9318/AD2298 coated on 2024-T3 Bare/Alodine 1200 panelshows some stains in the salt spray test (FIG. 2A) and thus gives a lowimpedance value of 4×10⁷ Ω·cm² (●) that is 100 times lower than thepainted AFP panel. A reduction in impedance value is also observed forthe painted Alodine 1200 panel after soaking in 3% NaCl solution for1000 hours (▪).

Throughout this application, author and year and patents by numberreference various publications, including United States patents. Fullcitations for the publications are listed below. The disclosures ofthese publications and patents in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the state of the art to which this invention pertains.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology that has been used is intended to bein the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the described invention, theinvention may be practiced otherwise than as specifically described.

1. A coating composition comprising a chrome-free environmentallyfriendly formulation.
 2. The coating according to claim 1, wherein saidcomposition includes water, resins, hazardous air pollutants-freeco-solvents, organofunctional silanes, metal chelating agents, andchrome-free corrosion inhibitors.
 3. The coating according to claim 2,wherein said composition further includes at least one pH adjustingagent.
 4. The coating according to claim 2, wherein said water ispresent in a range of 60-70% by weight, and preferably present in arange of 61-63% by weight.
 5. The coating according to claim 2, whereinsaid resin is present in a range of 15-25% by weight and preferably in arange of 20-23% by weight is preferred.
 6. The coating according toclaim 2, wherein said resin is of a size selected from the groupconsisting essentially of micro- and nano-particle size.
 7. The coatingaccording to claim 5, wherein said resin is selected from the groupconsisting essentially of an acrylic emulsion, a polyurethane emulsion,a co-polymer emulsion, and other similar compounds.
 8. The coatingaccording to claim 2, wherein said hazardous air pollutants (HAPs)-freeco-solvents are present in a range of 10-20% by weight and preferably ina range of 15-17% by weight.
 9. The coating according to claim 11,wherein said hazardous air pollutants (HAPs)-free co-solvents areselected from the group consisting essentially of DPnB and PnBco-solvents.
 10. The coating according to claim 2, wherein saidorganofunctional silanes are present in a range of 0.4-5% by weight andpreferably in a range of 1.5-2.5% by weight is preferred.
 11. Thecoating according to claim 10, wherein said organofunctional silanesinclude sterically hindered substituents located at silicon atoms. 12.The coating according to claim 11, wherein said substituents areselected from the group consisting essentially of vinyl, epoxy, sulfur,amino, functionalized mercaptosilanes, and aminosilanes.
 13. The coatingaccording to claim 2, wherein said corrosion inhibitors are present in arange of 0.1-1.0% by weight and preferably in a range of 0.3-0.5% byweight is preferred.
 14. The coating according to claim 11, whereincorrosion inhibitor is selected from the group consisting essentially ofsilicates, vanadates, metaborates, manganates, phosphates,mercapto-compounds, xanthic acid salts, dithiocarbamic acid salts,organic carboxylates, and other similar compounds.
 15. The coatingaccording to claim 3, wherein said composition includes trace amounts ofpH adjusting agents.
 16. The coating according to claim 15, wherein pHadjusting agent is selected from the group consisting essentially ofammonia, organic amines, and other similar agents.
 17. A metal alloycoated with the coating composition as set forth in claim
 1. 18. Ananti-corrosion coating comprising the composition set forth in claim 1.19. An anti-fingerprint coating comprising the composition set forth inclaim
 1. 20. A highly adhesive coating to the metal alloys andgalvanized steel as set forth in claim
 1. 21. A highly adhesive coatingto the subsequent liquid and powder paints as set forth in claim 1 22.The galvanized and galvalume coats with the coating composition as setforth in claim 1.